1. The Field of the Invention
The present invention relates generally to the field of optical data transmission systems. More particularly, the present invention relates to optoelectronic devices having a bias control loop to dynamically adjust the reverse bias applied to avalanche photodiodes during operation and thereby improve the operation of the avalanche photodiodes.
2. The Relevant Technology
Computer and data communications networks continue to develop and expand due to declining costs, improved performance of computer and networking equipment, the remarkable growth of the internet, and the resulting increased demand for communication bandwidth. Such increased demand occurs within and between metropolitan areas as well as within communications networks. Moreover, as organizations have recognized the economic benefits of using communications networks, network applications such as electronic mail, voice and data transfer, host access, and shared and distributed databases are increasingly used as a means to increase user productivity.
This increased demand, together with the growing number of distributed computing resources, has resulted in a rapid expansion of the number of fiber optic systems required. Through fiber optics, digital data in the form of light signals is formed by light emitting diodes or lasers and then propagated through a fiber optic cable. Such light signals allow for high data transmission rates and high bandwidth capabilities.
In a typical fiber-optic network, the transmission and reception of data is not strictly limited to optical signals, however. Digital devices such as computers may communicate using both electronic and optical signals. As a result, optical signals need to be converted to electronic signals and electrical signals need to be converted to optical signals. To convert electronic signals to optical signals for transmission on an optical fiber, a transmitter having a light emitter such as a laser is used. A transmitter uses an electronic signal to drive the light emitter to generate an optical signal. When optical signals are converted to electronic signals, a receiver is used. The receiver has a photodiode that, in conjunction with other circuitry, detects optical signals and converts the optical signals to electronic signals. A transceiver is a common device that incorporates both a transmitter and a receiver.
One very sensitive type of photodiode is an avalanche photodiode. Avalanche photodiodes are well-known devices that serve at least two functions: 1) conversion of optical signals into electrical signals; and 2) amplification of the electrical signal through avalanche multiplication. Typically, an avalanche photodiode has an absorption layer where an optical signal is absorbed. Photons in the optical signal impinging the absorption layer generate an electron-hole pair or a carrier. A multiplication layer in the avalanche photodiode is designed such that one carrier causes an avalanche of other carriers where the number of other carriers is dependent on the gain of the avalanche photodiode.
The gain of an avalanche photodiode is facilitated by a pre-applied electrical voltage across the avalanche photodiode, the result of which is that a reduced amount of power from an optical signal is required to trigger the “breakdown,” or the avalanche of carriers in the avalanche photodiode that results in the detection of an optical signal. This pre-applied electrical voltage is applied by a bias voltage in the circuitry driving the avalanche photodiode. It is preferable to raise the bias voltage to as near the breakdown level as possible without allowing the diode to go into breakdown. The amount the bias voltage is backed off from the breakdown level is sometimes referred to as the offset.
Though generally effective, present avalanche photodiodes suffer from various problems that reduce their longevity or increase their cost. For example, the correct bias voltage is a strong function of the temperature of the avalanche photodiode chip. Present avalanche photodiodes must therefore have their avalanche photodiode bias voltage calibrated over a range of temperature extremes to ensure proper operation. Still, thermal gradients within an optoelectronic device and component aging can render the calibration inaccurate.
In addition, some conventional avalanche photodiode systems have a problem in that the back off (or “offset”) in bias voltage is quite large. This is necessary since the control of the voltage versus temperature is crude and since it is important that the voltage never exceed the breakdown level. The large offset causes lower performance in the receiver. This can result in lower yield, relaxed (less competitive) specifications, and/or can require higher performance (higher price) components to reach the desired specifications.
As previously noted, the correct bias voltage is a strong function of the temperature of the avalanche photodiode chip. Various conventional products use a temperature sensor and a look-up table to set the bias. The values in this table are determined at manufacture time by placing the module in a temperature chamber and adjusting the avalanche photodiode bias voltage at cold, room, and hot temperatures. The performance of the avalanche photodiode improves as the bias voltage increases until the voltage becomes too high and the diode breaks down. Therefore, in devices using a temperature look-up scheme the voltage can be typically adjusted within about 2 volts or less of breakdown. Other products which use simpler control mechanisms are often set more than 5 volts below breakdown.
The conventional scheme using a temperature look-up table is disadvantageous, however, in that the temperature sensor is not located at the avalanche photodiode chip so any temperature gradient creates an error in setting the bias voltage. If the gradient is large enough (for example 10° C. in some designs), the diode can be driven into breakdown despite the use of the temperature look-up table. In addition, setting the table requires significant test time during manufacturing due to the need to calibrate the module at different temperatures. Also, once the module is calibrated, there is no allowance for aging or drift in the values of the many components which make up the avalanche photodiode system.